Method and apparatus for increasing the efficiency of a transmitter

ABSTRACT

The present invention relates generally to the field of RF transmitters, and more particularly to a method and apparatus for increasing the efficiency of transmitters which are capable of transmitting at different power levels in each of at least two frequency bands. An inventive method is presented as well as an inventive transmitter comprising at least one power amplifier in which the load in the transmission line is varied as the output power is varied, in order to keep the efficiency of the power amplifier at a high level.

This patent application claims priority from and incorporates byreference the entire disclosure of U.S. Provisional Patent ApplicationNo. 60/284,507, filed on Apr. 19, 2001.

FIELD OF THE INVENTION

The present invention relates generally to the field of RF technology,and more particularly to the field of RF transmitters.

BACKGROUND

In many applications of RF transmitters, it is desirable to introducethe possibility of the RF transmitter transmitting at different powerlevels. This is e.g. the case in mobile radio communications, where theoutput radio power of the mobile stations as well as of the basestations should advantageously be variable depending on signal qualityand distance between transmitter and receiver.

If the change in output power level is effectuated simply by changingthe input power level to the power amplifier of the RF transmitter, thepower amplifier will, at some power levels, have to work at combinationsof power supply voltage and input power level which yields poorefficiency of the power amplifier. The power supply voltage can beadjusted and the efficiency of the power amplifier optimised for thehighest of the power levels, while the efficiency will be impaired asthe input power level to the power amplifier is lowered. Since in manyapplications of RF transmitters, such as in mobile stations, one has torely on a battery as the power supply, the battery having limitedenergy, poor efficiency of the RF transmitter is a severe problem.

This problem has previously been solved by introducing a settableswitched power supply to the power amplifier, so that the power supplyvoltage can be lowered as the input power level to the power amplifieris lowered, the efficiency of the power amplifier thus staying at a highlevel. Another solution that has been found is to introduce atransformer with switched steps at the output of the power amplifier, sothat, without changing the input power level tot he power amplifier, theoutput signal power can be varied as different parts of the transformeris connected to the transmitter output at different times. However,there are certain drawbacks with each of these solutions. A settableswitched power supply generates disturbances which interferes with theRF signal to be transmitted. Further, it is difficult to designtransformers for ultra high frequencies covering large frequency ranges.As many RF transmitters today are designed for large frequency ranges,as in e.g. mobile radio telephony, where transmitters often are designedto operate in more than one frequency band, the frequency bands beingfar apart, the transformer solution is not optimal. Thus, it isdesirable to find a transmitter which can transmit RF signals atdifferent power levels in a wide frequency range without impairing theefficiency as the output power is changed, that does not introducedisturbances to the RF signal as the output power is changed.

SUMMARY

An object of the present invention is to provide a method and apparatusby which the transmission power level of an RF transmitter, capable oftransmitting in more than one frequency band, can be varied withoutsignificantly impairing the efficiency of the transmitter and withoutintroducing disturbances to the transmitted RF signal.

This object is met by transmitter for transmitting RF signals at atleast two power levels in each of at least two frequency bands. Thetransmitter comprises at least one power amplifier connected to aworking load and to a first electronic circuit. The first electroniccircuit is connected, in parallel to the working load, to the at leastone power amplifier at a line length from the at least one poweramplifier. The first electronic circuit comprises a reactive impedanceand a switching device connected in series, the reactive impedance andthe line length having been chosen as to provide, when the switchingdevice is closed, a load selected with regard to the efficiency of theat least one power amplifier when the transmitter transmits at one ofthe at least two power levels. If the transmitter comprises more thanone power amplifier, the line lengths at which the first electroniccircuit is connected to the power amplifiers is different for each poweramplifier.

The object of the invention is further met by an inventive method forimproving the efficiency of a transmitter upon changing the output powerlevel at which the transmitter transmits. The transmitter is capable oftransmitting at at least two power levels in each of at least twofrequency bands, and the transmitter comprises a power amplifierconnected to a working load. In the inventive method, at least oneelectronic circuit is connected, in parallel with the working load, tothe power amplifier at a line length from the power amplifier, theelectronic circuit comprising a switching device and a reactiveimpedance, the line length and the reactive impedance being chosen toform an optimal load with regard to the efficiency of the poweramplifier when the transmitter transmits in one of the at least twofrequency bands at one of the at least two power levels. If thetransmitter transmits at the power level and in the frequency band forwhich the line length and the reactive impedance has are chosen to forman optimal load, then said switching device is closed. If the RFtransmitter transmits at a power level and/or in a frequency band forwhich the line length and the reactive impedance are not chosen to forman optimal load, then said switching device is opened.

By the method and the transmitter of the invention is achieved that theload of a power amplifier of a transmitter can, in a simple manner,without introducing disturbances to the transmitted RF signal, beadjusted to the power level at which the transmitter transmits. Thus,the efficiency of the power amplifier and hence of the transmitter willnot have to be impaired by a change in the output power of thetransmitter.

In one embodiment of the inventive transmitter, the at least one poweramplifier is two separate power amplifiers, each operable on RF signalsin one of the at least two frequency bands. The two power amplifiers arein this embodiment connected to the first electronic circuit atdifferent line lengths. The reactive impedance of the first electroniccircuit and the two line lengths are selected in a manner as to providesaid load, when the switching device is closed, when the transmittertransmits in one of the at least two frequency bands. In one aspect ofthis embodiment, the reactive impedance is selected in a manner as toprovide said load, when the switching device is closed, no matter inwhich of the at least two frequency bands transmitter transmits. The atleast two power levels at which the transmitter is designed fortransmitting could then be the same for the at least two frequencybands, or different. The reactive impedance of the first electroniccircuit could in this embodiment e.g. be formed by a capacitor and aninductor connected in parallel or in series. Furthermore, the two poweramplifiers could be capable of operating on RF signals of differentmodulation modes.

In a second embodiment of the inventive transmitter, the at least onepower amplifier is one single power amplifier operable on RF signals ofthe at least two frequency bands, to which a second electronic circuitis connected. The second electronic circuit is connected in parallel tothe working load and the first electronic circuit, at a second linelength from the power amplifier. The second electronic circuit comprisesa second reactive impedance and a second switching device. The reactiveimpedance and the line length of the first electronic circuit areadjusted to provide said load selected with regard to the efficiency ofthe power amplifier when the dual mode RF transmitter transmits at oneof the at least two power levels in the first of the at least twofrequency bands. The second reactive impedance and the second linelength have been chosen to provide a load selected with regard to theefficiency of the power amplifier when the dual mode RF transmittertransmits at one of the at least two power levels in a second of the atleast two frequency bands. The power level with regard to which thereactive impedance and the line length of the first electronic circuithas been chosen and the power level with regard to which the reactiveimpedance and the line length of the second electronic circuit has beenchosen could be the same, or different.

In a third embodiment of the inventive transmitter, wherein the at leastone power amplifier is one single power amplifier operable on RF signalsof the at least two frequency bands, the transmitter comprises a filterarrangement connected at the output of the power amplifier. The filterarrangement provides different electrical lengths for the at least twofrequency bands. These electrical lengths, the line length and thereactive impedance of the first electronic circuit has been chosen in amanner as to provide said load, when the switching device is closed,when the transmitter transmits in at least one of the at least twofrequency bands. The electrical lengths, the line length and thereactive impedance of the first electronic circuit could have beenchosen in a manner as to provide said load, when the switching device isclosed, no matter in which of the at least two frequency bands thetransmitter transmits.

In one aspect of the invention, the line length(s) and the reactiveimpedance of the first electronic circuit have been chosen so that ratioof the load when the switching device is open and the load when theswitching device is closed equals the ratio of the power level for whichthe transmitter is designed to transmit when the switching device isclosed and the power level for which the transmitter is designed totransmit when the switching device is open.

In one aspect of the inventive method, the at least one electroniccircuit is one single electronic circuit and the dual mode RFtransmitter comprises two separate power amplifiers connected atdifferent line lengths to the electronic circuit. Each power amplifieris operable on RF signals in one of the frequency bands. The reactiveimpedance of the electronic circuit and the two line lengths areselected in a manner as to provide said load, when the switching deviceis closed, no matter in which of the at least two frequency bands thetransmitter transmits.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be discussed in more detail withreference to preferred embodiments of the present invention, given onlyby way of example, and illustrated in the accompanying drawings, inwhich:

FIG. 1 is a dual band RF transmitter for transmitting at two differentpower levels, the dual band RF transmitter comprising two poweramplifiers.

FIG. 2 is a dual band RF transmitter for transmitting at two differentpower levels, the dual band transmitter comprising one single poweramplifier.

FIG. 3 is another dual band RF transmitter for transmitting at twodifferent power levels comprising one single power amplifier.

DETAILED DESCRIPTION

In order to increase the efficiency of a power amplifier as the outputpower of the RF signal transmitted by an RF transmitter is varied, onecould use a variable load in the transmission path, additional to theworking load. When the power level of the RF signal which is fed intothe power amplifier is changed in order to change the power level of thetransmitted RF signal, the load of the transmission path could bechanged into a load for which the efficiency of the power amplifier iskept at a high value. The output voltage of the power amplifier isadvantageously kept constant as the transmitter output power level isvaried, while the load experienced by the power amplifier is varied. Theenergy consumed by the power amplifier would then be less than if the RFsignal which is fed into the power amplifier would be changed withoutadjusting the load.

An example of a dual band RF transmitter 100, or simply transmitter 100,operating according to the principle of varying the load in accordancewith at which power level the transmitter 100 is transmitting isschematically shown in FIG. 1, the transmitter 100 for transmitting RFsignals in two different frequency bands f₁ and f₂. These frequencybands could e.g. be 890 Mz to 915 MHz and 1710 MHz and 1785 MHz,corresponding to the uplink frequency bands of GSM (Global System forMobile Communication) and DCS (Digital Communication System), or anyother frequency bands. The dual band RF transmitter 100 of FIG. 1comprises two power amplifiers 105 and 110. Power amplifier 105 operateson RF signals in one of the two frequency bands, f₁, while poweramplifier 110 operates on RF signals in the other frequency band, f₂.Two transmission paths 115 and 120 connect power amplifier 105 and 110,respectively, to a combiner 125 which connects the two power amplifiers105 and 110 to the same output port. The line lengths l₁ and l₂ of thetransmission paths 115 and 120 could be adjusted separately. Thecombiner 125 could e.g. be a filter diplexer or a switch. The output ofcombiner 125 is then connected to an electronic circuit 130 and aworking load 135, the electronic circuit 130 and the working load 135being connected in parallel. The working load 135 could e.g. be anotheramplifier or an antenna, where the antenna could e.g. comprise anantenna array.

The electronic circuit 130 comprises a switch 140 connected in series toa reactive impedance, the reactance of the reactive impedance beingfrequency dependent and the reactive impedance thus providing differentreactances in the two frequency bands. In FIG. 1, a capacitor 145 ofcapacitance C₁₄₅ and an inductor 150 of inductance L₁₅₀ connected inparallel is used as an exemplary reactive impedance. The reactiveimpedance of electronic circuit 130 could alternatively comprise acapacitance and an inductance connected in series, or a more complexnetwork. When the switch 140 is open, the impedance that an RF signalexperiences at the output of power amplifier 105 or 110 is set by theimpedance of the working load 135, while when the switch 140 is closed,the impedance experienced by an RF signal will be determined by theimpedance of the working load 135 and the impedance of electroniccircuit 130.

The reflection coefficient ρ of a transmission path with characteristicadmittance Y₀ and a connected load of admittance Y_(L) is defined as:$\begin{matrix}{{\rho = {\frac{Y_{0} - Y_{L}}{Y_{0} + Y_{L}} = {\frac{1 - \frac{Y_{L}}{Y_{0}}}{1 + \frac{Y_{L}}{Y_{0}}} = \frac{1 - y_{L}}{1 + y_{L}}}}},} & (1)\end{matrix}$where y_(L) is the normalised load admittance. The impedance Zexperienced at an electrical length φ from the point where the load isconnected is then $\begin{matrix}{Z = {Z_{0}\frac{1 + {\rho\quad{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}}{1 - {\rho\quad{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}}}} & (2)\end{matrix}$where Z₀=1/Y₀ and the electrical length φ is measured in wavelengths ofthe RF signal on the transmission path expressed in angular terms.

In FIG. 1, the electronic circuit 130 and the working load 135 are bothconnected at the connection point 155. If the working load 135 isassumed to be only resistive, the admittance of the total load when theswitch 140 is closed, Y_(L) ^(closed), can be written as $\begin{matrix}{Y_{L}^{closed} = {{G_{WL} + {j\quad B_{130}}} = {G_{WL} + {j\left( {{\omega\quad C_{145}} - \frac{1}{\omega\quad L_{150}}} \right)}}}} & (3)\end{matrix}$where G_(WL) is the conductance of the working load, B₁₃₀ is thesusceptance of electronic circuit 130 and ω is the angular frequency ofthe RF signal. When the switch 140 is open, the admittance of the totalload Y_(L) ^(open) is simply{Y_(L) ^(open) =G _(WL)  (4)

If the impedance of the working load 135 has a reactive component,expressions (3) and (4) will have to be adjusted accordingly.

When designing the transmitter 100 for different load impedances atdifferent output power levels, the different load impedances areadvantageously chosen so that the output voltage of power amplifiers 105and 110 stays the same as the transmitter output power level is varied.The ratio of Z_(closed) and Z_(open) could hence advantageously equalthe ratio of the power levels P_(open) and P_(closed) for which theswitching device 140 is designed to be open and closed, respectively. Byinserting equation (3) and (4) into equation (1), equations for thereflection coefficients ρ_(closed) and ρ_(open) can be found,respectively. By inserting ρ_(closed) into equation (2), a value forZ_(closed) at an electrical length φ from the connection point 155 canbe obtained, while inserting ρ_(open) into the same equation similarlyyields an expression for Z_(open). If the admittance of the workingload, G_(WL), equals the characteristic admittance, Y₀, then ρ_(open)=0and the load experienced by the power amplifiers 105 and 110 when switch140 is open is Z₀, independent of the line lengths l₁ and l₂. Theexpression for the ratio Z_(closed)/Z_(open), set equal to the desiredvalue of P_(open)/P_(closed), is then: $\begin{matrix}{\frac{Z_{closed}}{Z_{0}} = {\frac{1 - {\frac{j\quad B_{130}}{{2Y_{0}} + {j\quad B_{130}}}{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}}{1 + {\frac{j\quad B_{130}}{{2Y_{0}} + {j\quad B_{130}}}{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}} = \frac{P_{open}}{P_{closed}}}} & (5)\end{matrix}$

Expression (5) is solved by two different sets of values of thesusceptance B₁₃₀ and electrical length φ. The power amplifiers 105 and110 each has an independent electrical length φ₁₀₅ and φ₁₁₀ to theconnection point 155, the electrical lengths being independentlyadjustable as the length of the transmission paths 115 and 120 arevaried. Furthermore, the susceptance B₁₃₀(ω) is a function of frequency.Since the power amplifiers operate on RF signals in different frequencybands, each power amplifier operating on RF power signals in only one ofthe frequency bands, the values of φ₁₀₅ and B₁₃₀(ω₁) as well as φ₁₁₀ andB₁₃₀(ω₂) can be chosen so that the value of Z_(closed)/Z_(open) staysthe same, no matter in which of the two frequency bands the transmitter100 is transmitting. By choosing C₁₄₅ and L₁₅₀ so that the resonancefrequency of electronic circuit 130 lies between the two frequency bandsf₁ and f₂, the susceptance B₁₃₀ could e.g. take one value at thefrequency band f₁, and another value, the modulus being the same but thesign being the opposite, at frequency band f₂.

For illustrative purposes, a numerical example will be given below,based on an exemplary dual band RF transmitter 100 operating in twoexemplary frequency bands centred around the frequencies 900 MHz and1800 MHz, respectively. The desired power level of the example, when theswitch 140 is closed, is half that of when the switch is open, i.e. thedesired value of ratio P_(open)/P_(closed) is 2. The characteristicadmittance Y₀ in the example is 0.02 Ω⁻¹. The RF signal travels on theexemplary transmission line with a speed of 0.6 c, where c is the speedof light. By inserting the values applicable to the example inexpression (5), one gets the following expression, where b=B₁₃₀/Y₀:$\begin{matrix}{2 = \frac{1 - \frac{j\quad b\quad{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}{2 + {j\quad b}}}{1 + \frac{j\quad b\quad{\mathbb{e}}^{{- 2}\quad j\quad\varphi}}{2 + {j\quad b}}}} & (6)\end{matrix}$

Expression (6) is satisfied by the following sets of b and φ:$\begin{matrix}\begin{matrix}{b_{1} = {- \sqrt{\frac{1}{2}}}} & \quad & \quad & \quad & {b_{2} = {+ \sqrt{\frac{1}{2}}}} \\{{\sin\quad 2\quad\varphi_{1}} = {+ \frac{2\sqrt{2}}{3}}} & \quad & \quad & \quad & {{\sin\quad 2\quad\varphi_{2}} = {- \frac{2\sqrt{2}}{3}}} \\{{\cos\quad 2\quad\varphi_{1}} = {- \frac{1}{3}}} & \quad & \quad & \quad & {{\cos\quad 2\quad\varphi_{2}} = {- \frac{1}{3}}} \\{\varphi_{1} = {{54.5{^\circ}} + {n\quad 180{^\circ}}}} & \quad & \quad & \quad & {\varphi_{2} = {{125.5{^\circ}} + {n\quad 180{^\circ}}}}\end{matrix} & (7)\end{matrix}$

In the calculations of the numerical example, one frequency in each ofthe two frequency bands is selected and inserted in the aboveexpressions: the frequency 900 MHz is used to represent the firstfrequency band, and 1800 MHz is used to represent the second frequencyband. By inserting the two values of the angular frequencies ω₁ and ω₂,corresponding to the two frequencies 900 MHz and 1800 MHz, into theexpression for b,${b = {{B_{130}/Y_{0}} = {\frac{1}{Y_{0}}\left( {{\omega\quad C_{145}} - {{1/\omega}\quad L_{150}}} \right)}}},$and setting the expression equal to the roots b₁ and b₂, one can obtainsuitable values for C₁₄₅ and L₁₅₀. In the example presently discussed,it is found that C₁₄₅ should be chosen to 2.5 pF and L₁₅₀ should bechosen to 6.3 nH.

By, in a similar manner, using the roots φ₁ and φ₂ of expression (7) andthe values of the frequencies f₁ and f₂, one can obtain values of theline lengths l₁ and l₂ that should be parting the power amplifiers 105and 100, respectively, and the connection point 155. Since the RF signalof the example is transmitted by 0.6 c, where c is the speed of light,the expression for l can be written as$l = {\frac{0.6c}{f}{\frac{\varphi}{360{^\circ}}.}}$It is then found that l₁, corresponding to the lower frequency band,should be 3.0 cm, while l₂, corresponding to the frequency band, shouldbe 3.5 cm. Any number of half wavelengths of the respective RF signalcould be added to these lengths.

The numerical example given above is merely an illustrative example usedfor illustration purposes, and the inventive transmitter of FIG. 1 couldbe used in any two frequency bands, with any ratio between the twooutput power levels and with any impedance on the working load. Thecharacteristic admittance Y₀ of the transmission lines could take anyvalue. Furthermore, the inventive transmitter shown in FIG. 1 could be adual mode transmitter, i.e. the power amplifiers 105 and 110 could notonly be operable on RF signals of different frequencies, but also ofdifferent modulation modes. Power amplifier 105 could e.g. be operableon an AM modulated signal of frequency f₁, while power amplifier 110 wasoperable on an FM modulated signal of frequency f₂. The capacitor 145and inductor 150 of electronic circuit 130 could be connected in seriesinstead of in parallel as shown in FIG. 1, or a more complex networkcould be used as the reactive impedance of electronic circuit 130.Expression (3) would then have to be adjusted accordingly.

The RF signals transmitted by transmitter 100 in frequency band f₁ couldeither be transmitted at the same power as the RF signals in frequencyband f₂, or at a different power. If the reactive impedance ofelectronic circuit 130 and the line lengths l₁ and l₂ are chosen so thatthe admittance Y_(L) ^(closed) in frequency band f₁ is the same as theadmittance Y_(L) ^(closed) in frequency band f₂, then the ratio betweenthe two power levels for which the loads are adjusted would be the same.One could also choose to select the reactive impedance of electroniccircuit 130 and the line lengths l₁ and l₂ in a manner so that theadmittance Y_(L) ^(closed) is different for the two frequency bands.

In FIG. 2, a dual band RF transmitter 200, or simply transmitter 200, isschematically shown, the transmitter 200 capable of transmitting RFsignals in two different frequency bands f₁ and f₂. Transmitter 200comprises a power amplifier 205, operable on RF signals in bothfrequency bands f₁ and f₂, connected to a working load 135. Connected inparallel to the working load 135 are two electronic circuits 215 and220, connected at the connection points 225 and 230 at a line length l₁and l₂ from the power amplifier, respectively. Electronic circuit 215comprises reactive impedance 235 of impedance X₂₃₅(ω) and a switch 240,while electronic circuit 220 comprises reactive impedance 245 ofimpedance X₂₄₅(ω)) and switch 250. The electrical lengths between theoutput of the power amplifier and the connection points 225 and 230,φ₂₂₅ and φ₂₃₀, could be adjusted independently since the line lengths l₁and l₂ could be chosen independently. The values of X₂₃₅(ω), X₂₄₅(ω),φ₂₂₅ and φ₂₃₀ could be chosen according to the principles of expressions(1) and (2) so that having switch 240 closed when transmitting infrequency band f₁ would give the same impedance experienced by the poweramplifier 205 as having switch 250 closed when transmitting in frequencyband f₂. Each electronic circuit 215 and 220 can hence be adjusted foroperating in one of the frequency bands, and the electronic circuits 215and 220 can be adjusted to provide the same, or similar, load when theswitch 240 (in case of f₁) or switch 250 (in case of f₂) is closed.Thus, by having both switches open, one load adjusted for transmissionat a first power level is obtained, and by having that switch closedwhich has been adjusted for the frequency band at which the transmitterpresently is transmitting, another load, adjusted for transmission at asecond power level, is obtained. Alternatively, one could design thetransmitter 200 for transmission at different power levels in the twofrequency bands f₁ and f₂, so that the reactive impedance 235 ofelectronic circuit 215 provides an impedance in the frequency band f₁which is different to the impedance provided by reactive impedance 245in the frequency band f₂.

In FIG. 3, a dual band RF amplifier 300 comprising only one poweramplifier 205 operable on RF signals in both frequency bands f₁ and f₂is shown. The RF dual band transmitter 300 comprises a filterarrangement 305, providing different line lengths for signals of the twofrequency bands f₁ and f₂. The filter arrangement 305 is connected inseries between the output of the power amplifier 205 and the workingload 135. At the output of the filter arrangement 305, an electroniccircuit 130, similar to the electronic circuit 130 in FIG. 1, isconnected in parallel to the working load 135. The exemplary filterarrangement 305 shown in FIG. 3 comprises a diplexer 310, which dividesthe incoming RF signal so that the part of the RF signal which lies inthe frequency band f₁ is transmitted on the transmission path 315, andthe part of the RF signal which lies in the frequency band f₂ istransmitted on the transmission path 320. The line lengths of thetransmission paths 315 and 320 could be adjusted independently tosatisfy the desired electrical lengths for the relevant frequency band,f₁ or f₂. The transmission paths 315 and 320 are then connected to acombiner 325, which connects the two transmission paths 315 and 320 tothe same output port. The filter arrangement 305 could e.g. be replacedby a single filter capable of providing different line lengths for thesignals of the two different frequency bands. By introducing filterarrangement 305 which provides different electrical lengths for RFsignals in the two frequency bands, it is achieved that despite havingone single power amplifier 205 operable on both frequency bands, onesingle electronic circuit 130, comprising one switching device connectedin series to a reactive impedance, can be used. In the exemplaryelectronic circuit 130 shown in FIG. 3, the reactive impedance is shownas a capacitor 330 connected in series to an inductor 335, but thereactive impedance of electronic circuit 130 could consist of aninductance and a capacitance being connected in parallel, or of a morecomplex network.

The capacitor 145 and inductor 150 of FIG. 1 could be replaced bysuitable lengths of line, or by any other arrangement which has asuitable impedance with the desired frequency dependence. The same wouldbe valid for capacitor 330 and inductor 335 of FIG. 3. Reactiveimpedances X₂₃₅(ω) and X₂₄₅(ω) of FIG. 2 could consist of capacitors andinductors connected in a suitable manner, or suitable lengths of line,or of any other arrangement which gives a reactive impedance of thedesired value. The switches 140, 240 and 250 could be diode switches,relays, or any other type of switching arrangement. The impedance ofelectronic circuit 130, together with value of the electrical lengthsφ₁₀₅ and φ₁₁₀, can be chosen so that a closed switch 140 gives either ahigher or a lower output power level than an open switch 140.Analogously, the values of X₂₃₅(ω), X₂₄₅(ω) and φ₂₂₅, φ₂₃₀ can be chosenso that a closed switch gives either a higher or a lower output powerlevel than an open switch. In FIGS. 1 and 2, the switches 140, 240 and250 have been shown to be connected between the power amplifier and thereactive impedances of the electronic circuits. The switches 140, 240and 250 could very well be positioned on the other side of the reactiveimpedances, between the reactive impedance of the relevant electroniccircuit and the ground.

The above discussed invention could be useful in many differentapplications of RF transmitters, among which dual band RF transmittersin dual band mobile stations is an important application. The exemplarytransmitters 100, 200 and 300 shown in FIG. 3 above are all dual band RFtransmitters capable of transmitting RF signals in two separatefrequency bands. It should however be understood that the invention isnot limited to dual band RF transmitters, but could be applied totransmitters capable of transmitting RF signals in more than twofrequency bands.

The invention could be used for transmission at other power levels thanthe power levels for which the load has been adjusted. If e.g. thetransmitter 100 of FIG. 1 would be used for transmission at a powerlevel that is not one of the power levels for which the load has beenadjusted, the switch 140 could either be open or closed. Whether theswitch should be open or closed could be made dependant on which of thetwo loads that would yield the best efficiency of the two poweramplifiers 105 and 110 at a particular power level. Similarly, theswitches 240 and 250 of FIG. 2 as well as switch 140 of FIG. 3 could beeither open or closed when transmitting at a power level which is notone of the power levels for which the load has been adjusted.

One skilled in the art will appreciate that the present invention is notlimited to the embodiments disclosed in the accompanying drawings andthe foregoing detailed description, which are presented for purposes ofillustration only, but it can be implemented in a number of differentways, and it is defined by the following claims.

1. A transmitter for transmitting RF signals at at least two powerlevels in each of at least two frequency bands, the transmittercomprising: at least one power amplifier connected to a working load; afirst electronic circuit connected, in parallel to the working load, tothe at least one power amplifier at a line length from the at least onepower amplifier; wherein the first electronic circuit comprises areactive impedance and a switching device connected in series, thereactive impedance and the line length having been chosen so as toprovide, when the switching device is closed, a load selected withregard to the efficiency of the at least one power amplifier when thetransmitter transmits at one of the at least two power levels; andwherein, if the at least one power amplifier is more than one poweramplifier, the line length at which the first electronic circuit isconnected to the power amplifiers is different for each power amplifier.2. The transmitter of claim 1, wherein: the at least one power amplifieris two separate power amplifiers, each of the two separate poweramplifiers being operable on RF signals in one of the at least twofrequency bands, the two separate power amplifiers being connected tothe first electronic circuit at different line lengths; and the reactiveimpedance of the first electronic circuit and the two line lengths areselected in a manner so as to provide said load, when the switchingdevice is closed, when the transmitter transmits in at least one of theat least two frequency bands.
 3. The transmitter of claim 2, wherein thereactive impedance of the first electronic circuit and the two linelengths are selected in a manner so as to provide said load, when theswitching device is closed, no matter in which of the at least twofrequency bands the transmitter transmits.
 4. The transmitter of claim3, wherein the at least two power levels are the same for the at leasttwo frequency bands.
 5. The transmitter of claim 2, wherein the twopower amplifiers are capable of operating on RF signals of differentmodulation modes.
 6. The transmitter of claim 1, wherein: the at leastone power amplifier is one single power amplifier operable on RF signalsof all of the at least two frequency bands; a second electronic circuitis connected, in parallel to the working load and the first electroniccircuit, to the power amplifier at a second line length from the poweramplifier, the second electronic circuit comprising a second reactiveimpedance and a second switching device; the reactive impedance and theline length of the first electronic circuit are adjusted to provide saidload when the transmitter transmits in the first of the at least twofrequency bands; and the second reactive impedance and the second linelength have been chosen to provide a load selected with regard to theefficiency of the power amplifier when the transmitter transmits at oneof the at least two power levels in a second of the at least twofrequency bands.
 7. The transmitter of claim 6, wherein the power levelwith regard to which the reactive impedance and the line length of thefirst electronic circuit has been chosen and the power level with regardto which the reactive impedance and the line length of the secondelectronic circuit has been chosen are the same.
 8. The transmitter ofclaim 1, wherein: the at least one power amplifier is one single poweramplifier operable on RF signals of all of the at least two frequencybands; a filter arrangement is connected at an output of the poweramplifier, the filter arrangement providing different electrical lengthsfor the at least two frequency bands; and said electrical lengths, theline length, and the reactive impedance of the first electronic circuitbeing chosen so as to provide said load, when the switching device isclosed, when the transmitter transmits in at least one of the at leasttwo frequency bands.
 9. The transmitter of claim 8, wherein theelectrical lengths, the line length, and the reactive impedance of thefirst electrical circuit being chosen so as to provide said load, whenthe switching device is closed, no matter in which of the at least twofrequency bands the transmitter transmits.
 10. The transmitter of any ofthe above claims, wherein: the reactive impedance of the firstelectronic circuit is formed by a capacitor and an inductors; and thecapacitor and the inductor are connected in parallel or in series. 11.The transmitter of claim 1, wherein the line length(s) and the reactiveimpedance of the first electronic circuit are chosen so that a ratio ofthe load when the switching device is open and the load when theswitching device is closed equals a ratio of the power level for whichthe transmitter is designed to transmit when the switching device isclosed and the power level for which the transmitter is designed totransmit when the switching device is open.
 12. The transmitter of claim1, wherein the working load is an antenna.
 13. The transmitter accordingto claim 1, wherein the transmitter is incorporated in a mobile station.14. A method for improving the efficiency of a transmitter upon changingthe output power level at which the transmitter transmits, thetransmitter being capable of transmitting at at least two power levelsin each of at least two frequency bands, the transmitter comprising atleast one power amplifier connected to a working load, the methodcomprising: connecting an electronic circuit, in parallel to the workingload, to the at least one power amplifier at a line length from thepower amplifier; wherein the electronic circuit comprises a switchingdevice and a reactive impedance, the line length and the reactiveimpedance being chosen to form an optimal load with regard to theefficiency of the at least one power amplifier when the transmittertransmits in one of the frequency bands at one of the at least two powerlevels; if the transmitter transmits at the power level and in thefrequency band for which the line length and the reactive impedance arechosen to form an optimal load, closing said switching device; and ifthe transmitter transmits at at least one of a power level and in afrequency band for which the line length and the reactive impedance arenot chosen to form said optimal load, opening said switching device. 15.The method of claim 14, wherein: the transmitter comprises two separatepower amplifiers, the two separate power amplifiers being connected atdifferent line lengths to the electronic circuit, each of the twoseparate power amplifiers being operable on RF signals in one of thefrequency bands; and the reactive impedance of the electronic circuitand the at least two line lengths are selected so as to provide the sameload when the switching device is closed, no matter in which of the atleast two frequency bands the transmitter transmits.
 16. The method ofclaim 14, wherein the reactive impedance is formed of a capacitor and aninductor connected in parallel or in series.
 17. The method of claim 14,wherein the at least one power amplifier is a single power amplifieroperating on all of the at least two frequency bands, the method furthercomprising: connecting a second electronic circuit, in parallel to theworking load and the first electronic circuit, to the power amplifier ata second line length from the power amplifier; wherein the secondelectronic circuit comprises a second switching device and a secondreactive impedance, the second line length and the second reactiveimpedance being chosen to form an optimal load with regard to theefficiency of the power amplifier when the transmitter transmits in asecond one of the frequency bands at one of the at least two powerlevels; if the transmitter transmits at the power level and in thefrequency band for which the second line length and the second reactiveimpedance are chosen to form an optimal load, closing said secondswitching device; if the transmitter transmits at at least one of apower level and in a frequency band for which the second line length andthe second reactive impedance are not chosen to form an optimal load,opening said second switching device.